PATH WALK II: Add --path-walk option to 'git pack-objects' #1819
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This will be helpful in a future change. Signed-off-by: Derrick Stolee <stolee@gmail.com>
In order to more easily compute delta bases among objects that appear at the exact same path, add a --path-walk option to 'git pack-objects'. This option will use the path-walk API instead of the object walk given by the revision machinery. Since objects will be provided in batches representing a common path, those objects can be tested for delta bases immediately instead of waiting for a sort of the full object list by name-hash. This has multiple benefits, including avoiding collisions by name-hash. The objects marked as UNINTERESTING are included in these batches, so we are guaranteeing some locality to find good delta bases. After the individual passes are done on a per-path basis, the default name-hash is used to find other opportunistic delta bases that did not match exactly by the full path name. The current implementation performs delta calculations while walking objects, which is not ideal for a few reasons. First, this will cause the "Enumerating objects" phase to be much longer than usual. Second, it does not take advantage of threading during the path-scoped delta calculations. Even with this lack of threading, the path-walk option is sometimes faster than the usual approach. Future changes will refactor this code to allow for threading, but that complexity is deferred until later to keep this patch as simple as possible. This new walk is incompatible with some features and is ignored by others: * Object filters are not currently integrated with the path-walk API, such as sparse-checkout or tree depth. A blobless packfile could be integrated easily, but that is deferred for later. * Server-focused features such as delta islands, shallow packs, and using a bitmap index are incompatible with the path-walk API. * The path walk API is only compatible with the --revs option, not taking object lists or pack lists over stdin. These alternative ways to specify the objects currently ignores the --path-walk option without even a warning. Future changes will create performance tests that demonstrate the power of this approach. Signed-off-by: Derrick Stolee <stolee@gmail.com>
The t0450 test script verifies that builtin usage matches the synopsis in the documentation. Adjust the builtin to match and then remove 'git pack-objects' from the exception list. Signed-off-by: Derrick Stolee <stolee@gmail.com>
The previous change added a --path-walk option to 'git pack-objects'. Create a performance test that demonstrates the time and space benefits of the feature. In order to get an appropriate comparison, we need to avoid reusing deltas and recompute them from scratch. Compare the creation of a thin pack representing a small push and the creation of a relatively large non-thin pack. Running on my copy of the Git repository results in this data: Test HEAD -------------------------------------------------------------- 5313.2: thin pack 0.00(0.00+0.00) 5313.3: thin pack size 589 5313.4: thin pack with --path-walk 0.00(0.00+0.00) 5313.5: thin pack size with --path-walk 589 5313.6: big pack 2.76(7.19+0.27) 5313.7: big pack size 14.0M 5313.8: big pack with --path-walk 5.76(6.72+0.16) 5313.9: big pack size with --path-walk 13.2M Note that the timing is slower because there is no threading in the --path-walk case (yet). The cases where the --path-walk option really shines is when the default name-hash is overwhelmed with collisions. An open source example can be found in the microsoft/fluentui repo [1] at a certain commit [2]. [1] https://github.com/microsoft/fluentui [2] e70848ebac1cd720875bccaa3026f4a9ed700e08 Running the tests on this repo results in the following output: Test HEAD -------------------------------------------------------------- 5313.2: thin pack 0.28(0.40+0.03) 5313.3: thin pack size 1.2M 5313.4: thin pack with --path-walk 0.07(0.06+0.00) 5313.5: thin pack size with --path-walk 18.4K 5313.6: big pack 4.05(29.48+0.41) 5313.7: big pack size 19.7M 5313.8: big pack with --path-walk 6.01(9.17+0.20) 5313.9: big pack size with --path-walk 16.5M Notice in particular that in the small thin pack, the time performance has improved from 0.28s to 0.08s and this is likely due to the improved size of the resulting pack: 18.4K instead of 1.2M. Finally, running this on a copy of the Linux kernel repository results in these data points: Test HEAD -------------------------------------------------------------- 5313.2: thin pack 0.03(0.02+0.00) 5313.3: thin pack size 4.6K 5313.4: thin pack with --path-walk 0.03(0.01+0.01) 5313.5: thin pack size with --path-walk 4.6K 5313.6: big pack 21.06(60.57+1.45) 5313.7: big pack size 158.3M 5313.8: big pack with --path-walk 37.65(57.83+0.67) 5313.9: big pack size with --path-walk 152.3M Signed-off-by: Derrick Stolee <stolee@gmail.com>
There are many tests that validate whether 'git pack-objects' works as expected. Instead of duplicating these tests, add a new test environment variable, GIT_TEST_PACK_PATH_WALK, that implies --path-walk by default when specified. This was useful in testing the implementation of the --path-walk implementation, especially in conjunction with test such as: - t0411-clone-from-partial.sh : One test fetches from a repo that does not have the boundary objects. This causes the path-based walk to fail. Disable the variable for this test. - t5306-pack-nobase.sh : Similar to t0411, one test fetches from a repo without a boundary object. - t5310-pack-bitmaps.sh : One test compares the case when packing with bitmaps to the case when packing without them. Since we disable the test variable when writing bitmaps, this causes a difference in the object list (the --path-walk option adds an extra object). Specify --no-path-walk in both processes for the comparison. Another test checks for a specific delta base, but when computing dynamically without using bitmaps, the base object it too small to be considered in the delta calculations so no base is used. - t5316-pack-delta-depth.sh : This script cares about certain delta choices and their chain lengths. The --path-walk option changes how these chains are selected, and thus changes the results of this test. - t5322-pack-objects-sparse.sh : This demonstrates the effectiveness of the --sparse option and how it combines with --path-walk. - t5332-multi-pack-reuse.sh : This test verifies that the preferred pack is used for delta reuse when possible. The --path-walk option is not currently aware of the preferred pack at all, so finds a different delta base. - t7406-submodule-update.sh : When using the variable, the --depth option collides with the --path-walk feature, resulting in a warning message. Disable the variable so this warning does not appear. I want to call out one specific test change that is only temporary: - t5530-upload-pack-error.sh : One test cares specifically about an "unable to read" error message. Since the current implementation performs delta calculations within the path-walk API callback, a different "unable to get size" error message appears. When this is changed in a future refactoring, this test change can be reverted. Signed-off-by: Derrick Stolee <stolee@gmail.com>
It can be notoriously difficult to detect if delta bases are being computed properly during 'git push'. Construct an example where it will make a kilobyte worth of difference when a delta base is not found. We can then use the progress indicators to distinguish between bytes and KiB depending on whether the delta base is found and used. Signed-off-by: Derrick Stolee <stolee@gmail.com>
Since 'git pack-objects' supports a --path-walk option, allow passing it through in 'git repack'. This presents interesting testing opportunities for comparing the different repacking strategies against each other. Add the --path-walk option to the performance tests in p5313. For the microsoft/fluentui repo [1] checked out at a specific commit [2], the results are very interesting: Test HEAD ---------------------------------------------------------- 5313.2: thin pack 0.41(0.48+0.03) 5313.3: thin pack size 1.2M 5313.4: thin pack with --path-walk 0.08(0.05+0.02) 5313.5: thin pack size with --path-walk 18.4K 5313.6: big pack 4.47(30.62+0.40) 5313.7: big pack size 19.6M 5313.8: big pack with --path-walk 6.76(9.87+0.23) 5313.9: big pack size with --path-walk 16.5M 5313.10: repack 96.87(664.29+2.75) 5313.11: repack size 439.5M 5313.12: repack with --path-walk 95.68(109.90+0.92) 5313.13: repack size with --path-walk 122.6M [1] https://github.com/microsoft/fluentui [2] e70848ebac1cd720875bccaa3026f4a9ed700e08 This repo suffers from having a lot of paths that collide in the name hash, so examining them in groups by path leads to better deltas. Also, in this case, the single-threaded implementation is competitive with the full repack. This is saving time diffing files that have significant differences from each other. A similar, but private, repo has even more extremes during repacking: Test HEAD ----------------------------------------------------------------- 5313.10: repack 2138.22(11961.00+17.67) 5313.11: repack size 6.4G 5313.12: repack with --path-walk 1351.46(1418.28+3.96) 5313.13: repack size with --path-walk 804.1M There are small benefits in size for my copy of the Git repository: Test HEAD ----------------------------------------------------------- 5313.10: repack 22.11(98.37+1.64) 5313.11: repack size 126.4M 5313.12: repack with --path-walk 66.89(75.61+0.58) 5313.13: repack size with --path-walk 109.6M As well as in the nodejs/node repository [3]: Test HEAD ------------------------------------------------------------- 5313.2: thin pack 0.01(0.01+0.00) 5313.3: thin pack size 1.6K 5313.4: thin pack with --path-walk 0.02(0.01+0.00) 5313.5: thin pack size with --path-walk 1.6K 5313.6: big pack 5.35(12.43+0.32) 5313.7: big pack size 52.2M 5313.8: big pack with --path-walk 7.12(11.97+0.27) 5313.9: big pack size with --path-walk 52.1M 5313.10: repack 87.74(342.90+4.24) 5313.11: repack size 739.7M 5313.12: repack with --path-walk 212.79(245.05+1.78) 5313.13: repack size with --path-walk 697.6M [3] https://github.com/nodejs/node This benefit also repeats in this instance in the Linux kernel repository: Test HEAD --------------------------------------------------------------- 5313.2: thin pack 0.04(0.00+0.03) 5313.3: thin pack size 4.6K 5313.4: thin pack with --path-walk 0.03(0.01+0.01) 5313.5: thin pack size with --path-walk 4.6K 5313.6: big pack 21.16(62.81+1.35) 5313.7: big pack size 158.3M 5313.8: big pack with --path-walk 36.09(55.25+0.67) 5313.9: big pack size with --path-walk 152.2M 5313.10: repack 734.26(2149.62+31.24) 5313.11: repack size 2.5G 5313.12: repack with --path-walk 1457.23(1618.15+7.00) 5313.13: repack size with --path-walk 2.2G It is important to see that even when the repository shape does not have many name-hash collisions, there is a slight space boost to be found using this method. Also, there is no known case where the space is worse with --path-walk. This is of course due to the second pass where all objects to be packed are sorted in the usual way and checked for deltas. This second pass is usually very fast as the path-walk has primed many objects with quality deltas that short-circuit other delta computation attempts. Signed-off-by: Derrick Stolee <stolee@gmail.com>
The t0450 test script verifies that the builtin usage matches the synopsis in the documentation. Update 'git repack' to match and remove it from the exception list. Signed-off-by: Derrick Stolee <stolee@gmail.com>
Users may want to enable the --path-walk option for 'git pack-objects' by default, especially underneath commands like 'git push' or 'git repack'. This should be limited to client repositories, since the --path-walk option disables bitmap walks, so would be bad to include in Git servers when serving fetches and clones. There is potential that it may be helpful to consider when repacking the repository, to take advantage of improved deltas across historical versions of the same files. Much like how "pack.useSparse" was introduced and included in "feature.experimental" before being enabled by default, use the repository settings infrastructure to make the new "pack.usePathWalk" config enabled by "feature.experimental" and "feature.manyFiles". Signed-off-by: Derrick Stolee <stolee@gmail.com>
Repositories registered with Scalar are expected to be client-only repositories that are rather large. This means that they are more likely to be good candidates for using the --path-walk option when running 'git pack-objects', especially under the hood of 'git push'. Enable this config in Scalar repositories. Signed-off-by: Derrick Stolee <stolee@gmail.com>
Previously, the --path-walk option to 'git pack-objects' would compute deltas inline with the path-walk logic. This would make the progress indicator look like it is taking a long time to enumerate objects, and then very quickly computed deltas. Instead of computing deltas on each region of objects organized by tree, store a list of regions corresponding to these groups. These can later be pulled from the list for delta compression before doing the "global" delta search. This presents a new progress indicator that can be used in tests to verify that this stage is happening. The current implementation is not integrated with threads, but could be done in a future update. Since we do not attempt to sort objects by size until after exploring all trees, we can remove the previous change to t5530 due to a different error message appearing first. Signed-off-by: Derrick Stolee <stolee@gmail.com>
Adapting the implementation of ll_find_deltas(), create a threaded version of the --path-walk compression step in 'git pack-objects'. This involves adding a 'regions' member to the thread_params struct, allowing each thread to own a section of paths. We can simplify the way jobs are split because there is no value in extending the batch based on name-hash the way sections of the object entry array are attempted to be grouped. We re-use the 'list_size' and 'remaining' items for the purpose of borrowing work in progress from other "victim" threads when a thread has finished its batch of work more quickly. Using the Git repository as a test repo, the p5313 performance test shows that the resulting size of the repo is the same, but the threaded implementation gives gains of varying degrees depending on the number of objects being packed. (This was tested on a 16-core machine.) Test HEAD~1 HEAD ----------------------------------------------------------------- 5313.2: thin pack 0.00 0.00 = 5313.3: thin pack size 589 589 +0.0% 5313.4: thin pack with --path-walk 0.00 0.00 = 5313.5: thin pack size with --path-walk 589 589 +0.0% 5313.6: big pack 2.84 2.80 -1.4% 5313.7: big pack size 14.0M 14.1M +0.3% 5313.8: big pack with --path-walk 5.46 3.77 -31.0% 5313.9: big pack size with --path-walk 13.2M 13.2M -0.0% 5313.10: repack 22.11 21.50 -2.8% 5313.11: repack size 126.4M 126.2M -0.2% 5313.12: repack with --path-walk 66.89 26.41 -60.5% 5313.13: repack size with --path-walk 109.6M 109.6M +0.0% This 60% reduction in 'git repack --path-walk' time is typical across all repos I used for testing. What is interesting is to compare when the overall time improves enough to outperform the standard case. These time improvements correlate with repositories with data shapes that significantly improve their data size as well. For example, the microsoft/fluentui repo has a 439M to 122M size reduction, and the repack time is now 36.6 seconds with --path-walk compared to 95+ seconds without it: Test HEAD~! HEAD ----------------------------------------------------------------- 5313.2: thin pack 0.41 0.42 +2.4% 5313.3: thin pack size 1.2M 1.2M +0.0% 5313.4: thin pack with --path-walk 0.08 0.05 -37.5% 5313.5: thin pack size with --path-walk 18.4K 18.4K +0.0% 5313.6: big pack 4.47 4.53 +1.3% 5313.7: big pack size 19.6M 19.7M +0.3% 5313.8: big pack with --path-walk 6.76 3.51 -48.1% 5313.9: big pack size with --path-walk 16.5M 16.4M -0.2% 5313.10: repack 96.87 99.05 +2.3% 5313.11: repack size 439.5M 439.0M -0.1% 5313.12: repack with --path-walk 95.68 36.55 -61.8% 5313.13: repack size with --path-walk 122.6M 122.6M +0.0% In a more extreme example, an internal repository that has a similar name-hash collision issue to microsoft/fluentui reduces its size from 6.4G to 805M with the --path-walk option. This also reduces the repacking time from 2,138 seconds to 478 seconds. Test HEAD~1 HEAD ------------------------------------------------------------------ 5313.10: repack 2138.22 2138.19 -0.0% 5313.11: repack size 6.4G 6.4G -0.0% 5313.12: repack with --path-walk 1351.46 477.91 -64.6% 5313.13: repack size with --path-walk 804.1M 804.1M -0.0% Finally, the Linux kernel repository is a good test for this repacking time change, even though the space savings is more reasonable: Test HEAD~1 HEAD ---------------------------------------------------------------- 5313.10: repack 734.26 735.11 +0.1% 5313.11: repack size 2.5G 2.5G -0.0% 5313.12: repack with --path-walk 1457.23 598.17 -59.0% 5313.13: repack size with --path-walk 2.2G 2.2G +0.0% Signed-off-by: Derrick Stolee <stolee@gmail.com>
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WIP
Thanks, -Stolee
Here is the range-diff from the equivalent patches in the previous version: